Recently, semiconductor elements such as IC and LSI are widely used in electronic devices and equipment to realize multifunctional applications of the equipment and to make the equipment light, small and handy. However, the use of many semiconductor elements makes the electronic device less resistant to disturbance from electric noises. The conventional method to protect the electronic devices from invading electric noises is to integrate by-pass capacitors such as film capacitor, laminated ceramic capacitor and semiconductor ceramic capacitor into the power line of IC and LSI. These capacitors display excellent performances to absorb low voltage noises and high frequency noises. They are, however, so impotent to high voltage pulses and electrostatic charges that mulfunctionings of the equipment, break-down of semiconductors and/or capacitors themselves occur frequently when the electronic equipment is invaded by high voltage pulses or electrostatic charges. Therefore, these technical problems in the conventional capacitors should be improved.
A new type capacitor, which has sufficient resistance and excellent absorbing ability to pulses as well as good noise absorbing ability and stability to temperature and frequency changes, was developed and disclosed in Japanese Laid-Open Patent Publication No. 57-27001 and Japanese Laid-Open Patent Publication No. 57-35303 etc., wherein varistor functions were added to a ceramic capacitor made of SrTiO.sub.3 series of ceramic materials and the capacitor was defined as "a semiconductor type ceramic capacitor having varistor functions" (hereinafter referred to as a ceramic capacitor with varistor functions). This ceramic capacitor with varistor functions works as a varistor when high voltage pulses and electrostatic charges invade while it absorbs low voltage noises and high frequency noises as a conventional capacitor, thereby protecting the electronic equipment and built-in semiconductors from abnormal high voltage noises, pulses and electrostatic charges generated by surrounding electronic equipment and devices, and providing wide application fields of the capacitor.
Since the electronic parts are made more sophisticated, lighter and smaller, and more handy, ceramic capacitors with varistor functions are also required to be smaller but higher in performance. The effective electrode area of the conventional single-plate type ceramic capacitor with varistor functions will be, however, decreased when the capacitor is miniaturized, resulting in the reduction of electric capacitance and hence inferior reliability of the capacitor. A capacitor having laminated structure of the electrode is devised to solve the aforementioned problem, wherein the effective electrode area is expected to be increased. However, the ceramic capacitor with varistor functions is conventionally manufactured by a process comprising a step of coating the surface of a SrTiO.sub.3 type semiconductor element with oxides, followed by a thermal diffusion process to form an electric insulating layer in the grain boundaries. It is recognized to be very difficult to manufacture "a laminate type ceramic capacitor having varistor functions" (hereinafter referred to as a laminated ceramic capacitor with varistor functions) by sintering the material of the ceramic capacitor simultaneously with the material of an inner electrode, though conventional laminated ceramic capacitors made from a BaTiO.sub.3 series of materials are manufactured by this process.
As a method to overcome the problem of simultaneous formation of a material of laminated ceramic capacitor with varistor functions by sintering the material of the ceramic capacitor together with the material of inner electrode, the following method for manufacturing a laminated ceramic capacitor with varistor functions is developed and provided, using methods as are disclosed in Japanese Laid-Open Patent Publication No. 54-53248 and Japanese Laid-Open Patent Publication No. 54-53250, comprising the steps of: printing a pattern corresponding to the inner electrode by using ceramic paste enriched in organic binder on the surface of the ceramic substrate; forming a porous sheet corresponding to the pattern of the inner electrode by sintering; and impregnating the porous sheet with electrically conducting metals under the appropriate pressure, or alternatively a step of forming a pattern for the inner electrode by a gilding or fusion method. These processes, however, involve many difficulties to be overcome and they are as yet far from practical applications.
Japanese Laid-Open Patent Publication No. 59-215701 provides a method comprising the manufacturing steps of: forming a raw sheet made of ceramic powder calcinated in a non-oxidizing atmosphere; printing a pattern of the inner electrode by using an electric conductive paste mixed with a thermal diffusion material on the surface of the raw sheet, said thermal diffusion material having the ability to form electric insulating layers in the grain boundaries; and sintering said raw sheet in an oxidizing atmosphere. Another method disclosed in Japanese Laid-Open Patent Publication No. 63-219115 comprises the manufacturing steps of: forming a raw sheet made of semiconductive ceramic powder as a main component, said main component being mixed with oxide to form an insulating layer and/or diffusion material containing a glass material; laminating said raw sheets with a plurality of inner electrode sheets mutually one by one; and sintering said laminated sheets in air or in an oxidizing atmosphere. In the above-discussed two manufacturing methods, however, sintering is performed at a relatively low temperature of 1000.degree.-1200.degree. C. Therefore, the capacitor elements thus formed are not made from a perfect sintered body of ceramic powder wherein grain boundaries are scarcely contacted with their faces. The ceramic capacitors obtained entail the shortcomings comprising: relatively small electric capacitance; small value of the voltage non-linearity coefficient .alpha. which is a representative characteristic of a capacitor having varistor functions; instability of varistor voltage; and inferior reliability as a capacitor. The manufacturing method disclosed in the latter invention, Japanese Laid-Open Patent Publication No. 63-219115, involves the step of adding a glass material to the ceramic powder which entails a problem of glassy phase deposits in the grain boundaries, whereby said electric characteristics tends to be poor and the capacitor element is not recognized as reliable. The manufacturing process is considered to be far from practical.
Concerning laminated capacitors having varistor functions, voltage non-linear laminated capacitor elements made of ZnO, Fe.sub.2 O.sub.3 and TiO.sub.2 series of ceramic materials have been provided in Japanese Laid-Open Patent Publication No. 58-23921. Capacitance of capacitor elements of this type are, however, very small and they involve the deficiencies in that they display little effect on the electric noises with voltage of below the varistor region or to high frequency noises, while they have excellent ability to absorb pulses and electrostatic charges of relatively high voltage.